1. Field of the Invention
The present invention relates to a polarization insensitive optical communication device utilizing optical preamplification and, more particularly, to such a device which uses polarization diversity to provide improved optical amplification.
2. Description of the Prior Art
In a conventional direct detection optical communication scheme, a message signal originates from a semiconductor light emitting source, travels over a length of optical fiber, and impinges the active region of a semiconductor photodetector. For many applications, this relatively simple system is satisfactory. However, at high bit rates (&gt;4 Gb/s, for example), the coupling efficiency of the system degrades significantly, with a sensitivity of only -26 dBm at 8 Gb/s transmission (with a 10.sup.-9 bit error rate (BER)). Most high bit rate systems require a sensitivity of at least -32 dBm. A solution to this problem is to provide optical amplification at the input of the photodetector. That is, preamplify the optical signal before it enters the photodiode. One method of achieving this preamplification is to transform the optical signal into an electrical form (with a conventional photodiode, for example), perform standard electrical amplification with any of the various methods well-known in the art, then reconvert the amplified electrical signal into an amplified optical signal at the input of the receiver photodiode. In theory, this is a workable solution. In practice, however, the need to perform these optical-electrical and electrical-optical conversions has been found to seriously degrade the quality of the message signal. Further, these systems often require rather sophisticated and expensive electrical components.
A preferable solution is to perform optical amplification directly upon the message signal. As discussed in the article "Wideband 1.5 .mu.m Optical Receiver Using Traveling-Wave Laser Amplifier", by M. J. O'Mahony et al. appearing in Electron Letters, No. 22, 1986 at pp. 1238-9, conventional lasers may be used to perform this optical amplification. Although this is considered an improvement, there still exists a problem with these devices in that they are sensitive to the state of polarization of the incoming light signal. In particular, due to the difference in confinement factors in the laser structure, the TE and TM polarization states may exhibit a difference in gain of approximately 10 dB. Such a polarization dependence is undesirable for optical amplifiers utilized with installed optical fiber-based communication networks, where the polarization state of the message signal is at best unknown, and at worst varies as a function of time.
Thus, a need remains in the prior art for achieving optical amplification which is truly polarization insensitive.